Internal combustion engines produce exhaust gases containing a variety of pollutants, including hydrocarbons, carbon monoxide, nitrogen oxides (“NOx”), sulfur oxides, and particulate matter. Increasingly stringent national and regional legislation has lowered the amount of pollutants that can be emitted from such diesel or gasoline engines. Exhaust systems containing various catalyst components have been developed to attain these low emission levels.
With the increasing complexity of these exhaust systems, on-board diagnostics have been developed to allow vehicle owners to understand the operating condition of the exhaust system. On-board diagnostics (“OBD”) in the context of a motor vehicle is a generic term to describe the self-diagnostic and reporting capability of the vehicle's systems provided by a network of sensors linked to a suitable electronic management system. Early examples of OBD systems would simply illuminate a malfunction indicator light if a problem were detected, but it provided no information on the nature of the problem. More modern OBD systems use a standardized digital connection port and are capable of providing information on standardized diagnostic trouble codes and a selection of real-time data, which enable rapid problem identification and resolution of a vehicle's systems.
Besides lowering engine emissions from a vehicle, newer legislation also requires the increasing use of on-board diagnostics (OBD) to notify the driver in case of a malfunction or deterioration of the emission system that would cause emissions to exceed mandatory thresholds; e.g., a particulate matter, carbon monoxide, hydrocarbon and/or NOx level above a set limit (typically in g/km of a pollutant emissions). A typical method of on-board diagnostics includes placing a sensor before a catalytic component and another sensor after the catalytic component and measuring the sensor value difference to determine whether the component is functioning properly. For example, U.S. Pat. No. 6,739,176 teaches a process for checking the operability of an exhaust gas purification catalyst for diesel engines, which includes placing CO sensors upstream and downstream of a catalyst as well as a temperature probe downstream of the catalyst. U.S. Pat. No. 4,029,472 discloses a thermoelectric sensor for detecting the quantitative content of combustibles in an exhaust gas having two thermocouple junctions where an exhaust gas oxidation catalyst adjacent one of the junctions provides a junction temperature differential when the ceramic body is exposed to exhaust gas flow. The output difference between the two junctions is proportional to the concentration of residual combustibles in the exhaust gas.
U.S. Pat. No. 8,127,537 discloses an exhaust system that comprises a three-way catalyst (TWC) and a single lambda sensor. The TWC has a catalyst composition in at least a part of the length of channels extending from the inlet end that has a reduced oxygen storage activity, or no oxygen storage activity, relative to the catalyst composition in the remainder of the TWC. The single lambda sensor is contacted substantially only with exhaust gas that has first contacted the TWC composition having a reduced oxygen storage activity or no oxygen storage activity. U.S. Pat. No. 8,205,437 teaches an exhaust system that comprises a monolith substrate coated with a catalyst, and a first sensor disposed in a hole defined in part by an external wall of the monolith substrate. The catalyst composition in at least a part of the length of channels extending from the upstream end has an increased activity for a reaction for which the catalyst composition is intended relative to catalyst composition in the remainder of the substrate. The sensor is arranged such that it is contacted substantially only with exhaust gas that has first contacted the catalyst composition having an increased activity. Also, U.S. Pat. No. 8,327,632 discloses an exhaust system which has a catalyzed soot filter (CSF), a control unit, and a catalyzed sensor. The system can increase hydrocarbon (HC) and/or carbon monoxide (CO) content in an exhaust gas flowing into the CSF resulting in combustion of the HC and/or CO in the CSF, a temperature increase of the CSF, and combustion of particulate matter collected on the CSF.
We have found that sensors placed upstream and downstream of a catalyst component are not accurate enough because of the transient nature of vehicle exhaust conditions, compounded by the relatively small exotherm typically found for diesel exhaust gas compared to the transient changes in exhaust gas temperatures.
As with any automotive system and process, it is desirable to attain still further improvements in on-board diagnostics systems. We have discovered a new on-board diagnostics system for a catalyzed substrate in an exhaust system.